Silicone release formulations are known and find utility in a variety of applications. Generally they are applied to a surface from either a solvent-based composition or a solvent-free composition. Solvent-based compositions typically result in cured coatings which exhibit a significantly lower coefficient of friction (COF) than do solvent-free compositions. While it is not fully understood why this is so, it is believed that it is at least partially due to the higher molecular weight of the segments between the functional groups present in the solvent-based compositions.
Solvent-free systems are usually based on relatively highly functionalized silicone polymers in the 2,000 to 30,000 molecular weight range and typically yield densely crosslinked, very rubbery feeling coatings. Formulations designed for solvent delivery, on the other hand, normally contain polymers with molecular weights in excess of 100,000 containing relatively fewer functional groups as the major silicone component. Cured coatings of these systems will have a much higher molecular weight between crosslinks, and the motion and flexibility of these long, unrestricted lengths of polydimethylsiloxane are believed to be largely responsible for the much more slippery feel of these coatings.
Typically, COF values for solvent-cast and solvent-free coatings are 0.05 and 0.40, respectively. These values are measured on a cured coating applied to polyethylene coated kraft paper at a coating weight of 1 g/m.sup.2.
For many applications, such as those requiring hand application of a transfer adhesive, higher COF is a serious disadvantage. For example, in such an application the user typically applies one face of the adhesive to a surface with a release liner in place on the second exposed adhesive surface. He or she then applies pressure to the release liner to secure the adhesive to the surface. The release liner may then be removed to expose the second face of adhesive.
In applying pressure to the release liner, the user often encounters resistance due to friction. If this resistance is too great, he or she may develop calluses or blisters on his or her fingers. It is desirable, therefore, to minimize this resistance. This may be accomplished by reducing the COF. While solvent-based compositions do provide a low COF, they require the use of special equipment to remove and recover the solvent. This undesirably adds to the complexity and cost of processing these systems.
Solvent-free systems, while not requiring the use of this special equipment, typically do not provide the low COF needed. Thus, they do not fulfill the need of the user.
A variety of approaches have been tried in order to obtain a low COF solvent-free silicone release coating. For example, Japanese KOKAI SHOWA 61-159480 (published Jul. 19, 1986) discloses a solvent-free, addition curable composition in which a few weight percent (wt %) of a high molecular weight vinyl functional silicone gum is incorporated in the formulation. For this approach to succeed, it is critical that the added gum incorporate a significantly lower level of functionality than the base polymer of the host formulation. If this condition is met, the gum apparently is preferentially expressed at the surface of the coating during the curing process leading to reductions in the COF.
Other approaches to reducing COF in addition curable formulations that have been described include the use of both branched siloxanes (see U.S. Pat. No. 5,082,951) and siloxanes having a trivinylsiloxy group at one end of the base polymer (see U.S. Pat. No. 4,870,149). The in situ generation of branched siloxanes in addition cure systems as a means of achieving reductions in COF, has been disclosed in Japanese KOKAI SHOWA 63-101453 and the in situ synthesis of siloxane gums has been disclosed in European Patent Publication No. 0 446 030 A2. The addition of vinyl functional gums to addition curable release formulations as cure accelerators has been disclosed in Re. 31,727.
As used in the prior art, a vinyl-functional polymer or gum is one in which the CH.sub.2 .dbd.CH-- groups are attached directly to the silicon atoms of the organopolysiloxane polymer or gum. Although the use of vinyl-functional gums does reduce the COF, it also reduces the rate of cure in higher alkenyl-functional systems.